CVE-2025-43859

h11 accepts some malformed Chunked-Encoding bodies

Severity Score

9.1

*CVSS v3.1

Exploit Likelihood

*EPSS

Affected Versions

*CPE

Public Exploits

*Multiple Sources

Exploited in Wild

*KEV

Decision

Attend

*SSVC

Descriptions

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

A flaw was found in the h11. This vulnerability allows request smuggling via improper parsing of chunked-coding message bodies, where h11 fails to validate the required \r
terminators.

Jeppe Bonde Weikop discovered that h11 incorrectly handled crafted HTTP requests. A remote attacker could possibly use this issue to smuggle malicious HTTP requests, which could potentially lead to security control bypass and information leakage.

*Credits: N/A

CVSS Scores

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Attack Vector

Network

Attack Complexity

Low

Authentication

None

Confidentiality

Complete

Integrity

Complete

Availability

None

* Common Vulnerability Scoring System

SSVC

Decision:Attend

Exploitation

None

Automatable

Yes

Tech. Impact

Total

* Organization's Worst-case Scenario

Timeline

2025-04-17 CVE Reserved
2025-04-24 CVE Published
2025-04-24 CVE Updated
2025-07-01 EPSS Updated
---------- Exploited in Wild
---------- KEV Due Date
---------- First Exploit

CWE

CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

CAPEC

References (4)

URL	Tag	Source
https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj	X_refsource_confirm
https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed	X_refsource_misc

URL	Date	SRC

URL	Date	SRC

URL	Date	SRC
https://access.redhat.com/security/cve/CVE-2025-43859	2025-06-05
https://bugzilla.redhat.com/show_bug.cgi?id=2362162	2025-06-05

Affected Vendors, Products, and Versions

Vendor		Product				Version		Other		Status
Vendor	Product	Version	Other	Status	<-- -->	Vendor	Product	Version	Other	Status
Python-hyper Search vendor "Python-hyper"		H11 Search vendor "Python-hyper" for product "H11"				< 0.16.0 Search vendor "Python-hyper" for product "H11" and version " < 0.16.0"		en		Affected